This invention relates to a magnetic tape data management method and data storage subsystem.
Recent years have seen an expansion in the scale of computer systems, along with a rapid increase in the amount of data stored by such systems. In a computer system, various types of data are stored in external storage subsystems employing magnetic disks, magnetic tape and other types of storage media. As the amount of storage and data increases, the number of external storage subsystems and the number of storage media used by the system both increase, generating more system management and operational work, raising costs, increasing the amount of space occupied by the system, and giving rise to various other problems. Being a storage medium with high portability, magnetic tape has experienced a particularly large increase in utilization as a backup medium for data stored on magnetic disks and as a medium for permanent data storage. The amount of magnetic tape used in computer systems has thus been increasing steadily. The aforesaid problems are further aggravated by the fact that progress in increasing medium storage capacity has been slower for magnetic tape than for magnetic disks, which have already achieved high capacity levels.
One solution to these problems is to use digital audio tape recorders (DATs) or digital video tape recorders (VTRs) as computer external storage subsystems (see Tada and Fushiki, "Format For Using DAT For Recording Code Data, a proposal made jointly by Sony and Hewlett-Packard" Nikkei Electronics Mar. 7, 1988, No. 442, pp. 145-148, Fushiki, "One G-Byte Magnetic Tape Subsystem for External Storage Using DAT Tape", Nikkei Electronics, Jun. 15, 1987, No. 423, p. 77, Suzuki, Oyama, and Yoshizawa "Next-Generation VTR To Use Digital Recording" Nikkei Electronics, May 30, 1988, No. 448, pp 111-137, particularly page 128.)
Compared with the magnetic tape conventionally used for computer external storage, DAT and digital VTR recording media have the advantages of higher capacity, smaller size and lower cost. Digital VTR tape is specially advantageous. While about twice the size of the small cartridge type magnetic tape used in general purpose computers, it has about 100 times the recording capacity and, with a cost per unit capacity that is 10 to several 10s of times lower, is extremely inexpensive. Because of these features, the use of digital VTRs as magnetic tape subsystems for external computer storage is expected to substantially increase.
However, the following problems arise when conventional magnetic tape subsystem technology is applied without modification to digital VTRs.
Since the nature of magnetic tape subsystems requires them to employ sequential or serial access, read/write operations to the magnetic tape have to be carried out sequentially in the forward or reverse direction. Therefore, when the program of the host data processing unit wants to access data at an intermediate or middle portion of the magnetic tape, the fact that access is sequential makes it necessary for the magnetic tape subsystem to find the beginning of the particular data area concerned by starting at the beginning of the tape and sequentially positioning the tape with respect to the magnetic head at the magnetic tape marks written at the beginning and end of each data area, skipping over the data areas between the beginning of the magnetic tape and the data area desired. However, since the capacity of digital VTR tape is large and many data areas are apt to be stored on each reel, a long time is required for accessing specific areas by the conventional technique.
In the case of a digital VTR tape, it is possible to carry out a high speed search by using identifiers for specifying physical positions on the magnetic tape. The aforementioned problem can thus be overcome by storing data referring to positions on the magnetic tape as magnetic tape directory data and using the position data for rapidly locating a position of the desired data.
Methods usable for storing the directory data containing tape position data include one in which the program of the host data processing unit manages the directory data and one in which the directory data is stored in memory on the storage subsystem using a separate digital VTR for storing only directory data. The first method has the drawback of requiring a special digital VTR interface and the drawback of involving read/write to a digital VTR connected with another data processing unit, while the second method is disadvantageous in that it requires read/write to tape in a separate digital VTR.
While it is preferable to use a method in which the directory data is recorded on the data storage medium itself, recording the directory data at a specific single place on the magnetic tape makes it necessary to access this part of the tape every time the directory data is to be updated or referred to. This gives rise to performance and reliability problems. Further, a digital VTR magnetic tape cassette can be loaded/unloaded relative to the magnetic tape subsystem without rewinding the tape back to the beginning. Therefore, with the aforesaid method of recording the directory data at a specified single place, considerable time is required for locating the directory after the magnetic tape is loaded in the magnetic tape subsystem.
For overcoming these problems, it has been known to store data related to the magnetic tape on an IC card or the like coupled with the recording medium as set out in Japanese Laid-Open in which an electronic memory circuit for storing position data relative to a cassette-type magnetic tape is provided integrally within the cassette.
The device disclosed in the above-mentioned Japanese Laid-Open Application No. 55-58862 requires an IC card or other such electronic memory circuit to be affixed to every cassette or medium and also requires a circuit in the magnetic tape storage subsystem for connection with the electronic memory circuit and/or read/write of data. This tends to increase the overall size and complexity of the subsystem.
On the other hand, as regards the method in which directory data containing position data is recorded on the medium (magnetic tape), the following problems arise:
(1) As was explained earlier, when the directory data is recorded at a specific single place on the magnetic tape, this place has to be accessed every time the directory data is to be updated. Further, a digital VTR magnetic tape cassette can be loaded/unloaded relative to the magnetic tape subsystem without rewinding the tape back to the beginning. Thus, once the magnetic tape has been loaded in the magnetic tape subsystem, it has to be wound forward or backward to the specified place at which the directory data is recorded. This takes considerable time. The first problem with recording directory data at a specified single place on the magnetic tape is thus that a long access time is required for directory data updating or reference.
(2) As digital VTR magnetic tape is inferior in durability to the magnetic tape conventionally used in computer systems, it tends to deteriorate quickly at portions where repeated read/write operations are conducted. Since recording of the directory data at a specific single portion of the tape leads to many read/write operations being conducted at this portion, it may become impossible to read/write directory data there. In such a case, the impossibility of reading or writing directory data makes it impossible to access the data recorded at other portions of the tape even though normal read/write of data is possible in the physical sense. Thus, the second problem with recording directory data at a specified single place on the magnetic tape is that the reliability of the system becomes highly dependent on the durability of the tape.
(3) If the directory data is recorded at a plurality of places on the magnetic tape, the place at which new directory data is recorded will not be the only place containing directory data since other portions where old directory data is recorded also exist on the tape. Therefore, when the magnetic tape is loaded in a different magnetic tape subsystem from the one used for recording directory data or when read/write is conducted by a magnetic tape subsystem that becomes faulty before the recording of directory data, the tape may be erroneously positioned at a place recorded with the wrong (not the latest) directory data.
(4) When data regarding positions on the magnetic tape and the like are recorded as directory data, the amount of such directory data becomes voluminous owing to the large capacity of a digital VTR magnetic tape. As a result, a long time is required for the directory data input/output processing.
(1) The durability of digital VTR magnetic tape being inferior to that of conventional tape used in a computer system, portions of the tape which repeatedly come in contact with the magnetic head for read/write of not only directory data but also of ordinary data tend to deteriorate quickly.
To overcome the aforesaid problems without a need for adding complex circuits, the invention has the following objects.
The first object of the invention is to provide a method of recording directory data on a magnetic tape which reduces the access time during updating (writing) and reference to (reading) the directory data.
The second object of the invention is to provide a method of recording directory data on magnetic tape which makes it possible to read/write with respect to portions of the magnetic tape at which directory data is recorded for approximately the same time (tape service life) normally possible to read/write with respect to other portions of the magnetic tape.
The third object of the invention is to provide a method of recording directory data on a magnetic tape which makes it possible to position the tape with respect to the read/write head accurately at the portion of the tape where the latest valid directory data is stored even though directory data is recorded at a plurality of places on the tape. References to "positioning the tape" in this specification should be understood to refer to positioning of the tape with respect to a magnetic tape head.
The fourth object of the invention is to shorten the time for input/output processing of directory data by reducing the amount of such data.
The fifth object of the invention is to prevent rapid deterioration of the magnetic tape by insuring uniform use of all portions thereof.
1) For achieving objects one and two mentioned above, the invention provides the following methods of recording directory data:
a. After accessing data on the magnetic tape, directory data including data regarding the position of data on the tape is recorded in a directory data region (DDR) of the tape near the place on the tape at which the magnetic head is positioned at the time processing of data has been completed. In the preferred embodiment "near" means "closest to and after". "Closest to an after" refers to the nearest available DDR downstream on the tape (opposite of the direction of travel of the tape). Remember, the head axis is fixed (even if rotary) and the tape moves past the head so "after" is defined by position, relative to the fixed head, of the tape. In the preferred embodiment, the nearest DDR is immediately after the head location when processing is finished regardless of whether the tape is preformatted with allocated DDRs or whether DDRs can be written on any available tape space. Of course, reverse reading and writing is possible so "closest to and before" is also envisioned as within the scope of the present invention in addition to "closest to and after". PA1 b. A memory for recording directory data is also provided in the magnetic tape subsystem, removed from the tape itself, and directory data relating to the magnetic tape is recorded there in response to completion of data processing. Before the magnetic tape is unloaded from the magnetic tape subsystem, the directory data relating to the tape is read from the directory data memory in the magnetic tape subsystem and is recorded in a region of the tape "near" the place at which the magnetic tape is positioned just before unloading the tape. In the preferred embodiment, "near" means closest to and after. When the magnetic tape is loaded in the magnetic tape subsystem, the latest directory data relating to the tape is read from the region on the tape where it is stored and written to the directory data memory in the subsystem. PA1 c. The region of the magnetic tape for recording the directory data is defined as any free tape area near the place where the magnetic is positioned at the time of concern when the tape is used with no predefined areas or as one of a plurality of directory data regions established on the magnetic tape near the place at which the magnetic tape is positioned at the time concerned when the tape is initialized or formatted with specific DDRs. In either case, the region in which the directory data is recorded is the closest such region after (or before) the data position. PA1 a. In the case of a cassette-type magnetic tape and subsystem which allows the tape to be loaded/unloaded with respect to the magnetic tape subsystem without rewinding the tape to the beginning, unloading of the tape from the magnetic tape subsystem is carried out with the tape positioned such that the magnetic tape head is positioned at the end of the latest directory data recorded in the directory data region. At the time of loading of the magnetic tape, the magnetic tape is rewound by the length of the directory data region to position it at the beginning of the directory data region (or the DDR can be reverse read without rewinding). PA1 b. In the case of a magnetic tape subsystem equipped with a circuit for detecting tape position data of a specific pattern recorded on the tape, the detection being carried out when the tape travels at a high speed (the second speed) that is faster than the tape speed during data transfer (the first speed) the specific pattern data is recorded immediately before the directory data region on the tape. For positioning at the directory data region, the specific pattern data located immediately before the directory data region concerned is detected while the tape is being driven at the high second speed, whereafter positioning at the directory data region is effected. PA1 a. A region for a LATEST DATA FLAG is provided within the directory data region on the tape for indicating that the directory data in the region is the latest. When the directory data recorded in the directory data region of the tape is referred to (read), the latest data flag within the directory data region is reset, and when the latest directory data is recorded in the directory data region, the latest data flag with in the directory data region is set. PA1 b. A region for an UPDATE COUNTER indicating the number of directory data updates is provided within the data region on the magnetic tape and the update counter value is incremented when directory data is recorded in the directory data region. PA1 c. When the magnetic tape is loaded in the magnetic tape subsystem, the directory data region of the tape in which the latest directory data is stored is designated by the aforesaid LATEST DATA FLAG and when no directory data region of the tape has a LATEST DATA FLAG that is set, the directory data region in which the latest directory data is stored is determined from the UPDATE COUNTER value. PA1 d. In the case where a memory for recording directory data is provided in the subsystem, the memory will be nonvolatile and will not lose the data should the power to the magnetic tape subsystem be cut off. PA1 a. The data storage region on the magnetic tape can be partitioned into a plurality of sub-regions. When a data storage region on the magnetic tape is partitioned into a plurality of sub-regions, the invention provides both a LATEST DATA FLAG for the entire magnetic tape (first LATEST DATA FLAG) and a LATEST DATA FLAG for each sub-region (second LATEST DATA FLAG). Also the invention provides both an UPDATE COUNTER for the entire magnetic tape (first UPDATE COUNTER) and an UPDATE COUNTER for each sub-region (second UPDATE COUNTER). When data on the tape loaded in the subsystem is accessed, a first type of directory data relating to the entire tape and a second type of directory data relating to the sub-region concerned is, in response to the completion of data process at the individual sub-region concern, recorded in a region near the place at which the magnetic tape head is positioned at the time the processing of data on the tape is completed. PA1 c. The first type of directory data includes magnetic tape identifier and attribute data, magnetic tape use mode data and data relating to the position of the tape sub-regions and free areas. The second type of directory information includes identifier and free area data for the sub-regions and identifier, position and attribute data or data recorded in the sub-regions.
The specification and claims refer to data directory regions and data regions. It is important to define "region" as either a predetermined region having a fixed length or as a randomly selected, random length region on any portion of the tape. Whether the tape is preformatted or initialized with dedicated regions or whether no dedicated regions exist, the area where data and directory data are written can be defined as a "region".
(2) For achieving the first object of mentioned above, the invention further provides the following methods of positioning, with respect to the read/write head, the region of the magnetic tape for recording directory data.
The position of the specific pattern is set to be located ahead of the directory data region by a distanced equal to or greater than the distance travelled by the magnetic tape during the time required, following detection of the specific pattern data by the magnetic tape subsystem, for the tape travel speed to change from the second speed to the first speed and for data transfer to begin.
(3) For achieving the third object mentioned above, the invention further provides the following methods of determining the directory data region on the magnetic tape in which the latest directory data is recorded.
In the case of a cassette-type magnetic tape and a cassette-type magnetic tape subsystem which allows the tape to be loaded/unloaded with respect to the subsystem without rewinding the tape to the beginning, unloading of the tape is carried out with the read/write head at the end of the directory data region. At the time of loading of the magnetic tape, the magnetic tape is rewound by the length of the directory data region to find the directory data region whose LATEST DATA FLAG is set.
(4) For achieving the fourth object mentioned above, the invention provides the following methods for directory management.
b. Instead of causing magnetic tape directory data to be recorded in a region of the magnetic tape every time processing of data on the tape is completed, a memory for recording directory data is located in the magnetic tape subsystem for retaining directory data until it is needed for writing to the tape. A first type of directory data relating to the entire tape and a second type of directory data relating to the sub-region concerned is, in response to the completion of data processing at the sub-region, recorded in the directory data memory in the subsystem. When the magnetic tape is unloaded from the magnetic tape subsystem, the first type of directory data relating to the entire tape and the second type of directory data related to the sub-region at which the tape head is positioned just before the tape is unloaded are read from the directory data storage memory of the magnetic tape subsystem and recorded in a region of the tape near (closest to and after, in the preferred embodiment) the place at which the magnetic tape head is positioned just before unloading. When the magnetic tape is loaded, the magnetic tape is positioned at the directory data region where the latest directory data for the entire magnetic tape is stored based on the first LATEST DATA FLAG and the first UPDATE COUNTER, and the latest directory data is read therefrom and written to the directory data storage memory remote from the tape. When the magnetic tape is driven to position it at a second sub-region different from the first sub-region position at loading, a first type of directory data relating to the entire tape and a second type of directory data relating to the first sub-region are recorded in free regions closest to both the place at which the magnetic tape head was positioned before the tape was driven and nearest the place at which the magnetic tape head is positioned after the tape is driven. Two directories are written, one in each sub-region. When no directory data relating to the second sub-region is recorded in the memory of the magnetic tape subsystem, the second type of directory data in the latest directory data for the second sub-region is read from the second sub-region storing the same and written to the directory data storage memory of the magnetic tape subsystem.
(5) For achieving the fifth object mentioned above, the invention partitions the data storage regions on the magnetic tape into sub-regions of the same fixed length. Data is stored relating to the durability of the tape at the individual fixed-length sub-regions, providing the tape with directory data relating to the number of tape passes at the individual sub-regions.
Since the invention does not require the directory data to be recorded at a specific part of the magnetic tape, it reduces the amount of tape winding and rewinding and shortens access time. In fact, since directory data regions need not be specified in advance, this data management system takes on some of the characteristics of a random access memory of the magnetic disk type.
Since the directory data is recorded near the accessed data in response to the completion of each data processing operation ("data processing operation" can mean a single data accessing or a series of accesses, after which directory data is written), the frequency of reading and writing at the parts of the tape where directory data is recorded is approximately the same as at other parts of the tape. The service life of the parts of the tape where directory data is recorded thus become about the same as that of the other parts.
In a configuration having a memory for recording directory data established in the subsystem, directory data does not have to be recorded on the magnetic tape after completion of an ordinary data processing operation so the amount of time required for recording directory data is further shortened and the probability of it becoming impossible to read/write in places at which directory data is recorded before it becomes impossible to read/write with respect to the other places, is reduced.
In the system using a cassette-type magnetic tape and a cassette-type subsystem which allows the tape to be loaded/unloaded with respect to the subsystem without rewinding the tape to the beginning, upon loading the magnetic tape in the magnetic tape subsystem, the tape can be rewound by the length of the directory data region to position it at the beginning of the DDR. Therefore, the time required for positioning the tape at the DDR is short. Further, when using a subsystem capable of conducting high speed searches for specific pattern data, since positioning at the directory data region can be achieved by a rapid search for the specific pattern data recorded immediately before the directory data region, the time required for positioning at the proper DDR is, once again, short.
Since the invention provides a LATEST DATA FLAG in the DDR on the magnetic tape, it is possible to position the tape where the latest directory data is recorded when the tape is reloaded in the magnetic tape subsystem after being once unloaded therefrom. Moreover, since the invention provides a directory data UPDATE COUNTER in the directory data region on the magnetic tape, it is possible to use the UPDATE COUNTER value for positioning at the region where the latest directory data is recorded, even in the case where a failure arose in the magnetic tape subsystem before the directory data was updated.
When the system partitions a data storage region on the tape into a plurality of sub-regions, it divides the directory data into a first type of directory data relating to the entire tape and a second type or directory data relating to a specific sub-region, and records the first and second type of directory data relating to the sub-regions in individual directory data regions on the tape. As a result, the amount of directory data recorded in the individual directory data regions on the tape is reduced, which, in turn, shortens the time required for input/output of directory data.
Further, the invention can partition the data storage regions on the tape into sub-regions of the same fixed length and can manage the system using the number of passes of the magnetic tape at the individual sub-divisions as directory data relating to the durability of the tape, thereby enabling tape region use that equalizes the number of passes among the different parts of the tape, thus preventing rapid deterioration of the tape.
The invention thus enables a digital VTR or other such high capacity memory subsystem to be used as the external storage subsystem of a computer system.